1. Technical Field of the Invention
The present invention relates generally to a display device including a matrix of light-emitting elements which are selectively turned on and off to display a still and a moving picture, and more particularly to a display device designed to compensate for differences in luminance and chromaticity of light between pixels for producing high-quality images without irregularity in color over the whole of a screen.
2. Background Art
There are known display units which change the brightness or luminance and chromaticity of light produced by a discharge tube, a CRT, or an array of point sources such as light-emitting diodes (LEDs) each defining a pixel on a screen in response to an image signal to form a still image or a moving image.
LEDs used as light sources defining pixels of a screen are superior in reliability and lifetime to the discharge tubes and CRTs and are employed, especially as pixels on a large-sized screen in recent years. For example, a display unit is known which defines a screen with a rectangular array of pixels each made up of primary color LEDs: red, green, and blue LEDs and modifies the luminance of light emitted by each of the LEDs in response to an image signal to produce a full-color still picture or moving picture.
The color of each pixel of the picture is produced by controlling the brightness of the primary color LEDs in given proportions. However, even when the same brightness level signals are provided to the LEDs, a difference in chromaticity may arise on the screen, which will lead to irregularity in color of the image, resulting in a decrease in image quality.
FIG. 8 shows the CIE 1931 standard colorimetric system (XYZ). As can be seen from the drawing, even when LEDs of the same color are actuated, irregularity in color of light emitted from the LEDs is visually perceived because of a difference in chromaticity between the LEDs. Specifically, an increase in the chromaticity difference between the LEDs will also cause a difference in chromaticity between pixels each consisting of the LEDs of primary colors to be produced.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a display device which is designed to compensate for at least one of differences in luminance and chromaticity of light between pixels for producing high-quality images without irregularity in color over the whole of a screen.
According to one aspect of the invention, there is provided a display device. The display device includes: (a) a screen defined by a matrix of pixels on which an image represented by an image input signal is to be displayed; (b) first light-emitting elements one for each of the pixels, each of said first light-emitting elements being actuated to produce light of a first color in a corresponding one of the pixels; (c) second light-emitting elements one for each of the pixels, each of said second light-emitting elements being actuated to produce light of a second color different from the first color in a corresponding one of the pixels; and (d) a light-emitting element controlling circuit responsive to the image input signal to control actuation of each of said first and second light-emitting elements to form the image on said screen, said light-emitting element controlling circuit actuating said second light-emitting element in each of the pixels to visually mix the light of the second color with the light of the first color produced by a corresponding one of said first light-emitting elements in a given proportion to minimize a difference in chromaticity of the lights of the first color between the pixels.
In the preferred mode of the invention, the light-emitting element controlling circuit also actuates said first light-emitting element in each of the pixels to visually mix the light of the first color with the light of the second color produced by a corresponding one of said second light-emitting elements in a given proportion to minimize the difference in chromaticity of the lights of the second color between the pixels.
Third light-emitting elements may be provided one for each of the pixels. Each of said third light-emitting elements is actuated to produce light of a third color different from the first and second colors. The light-emitting element controlling circuit actuates said second light-emitting element and said third light-emitting element in each of the pixels in a given luminance proportion to minimize a shift in chromaticity of the light of the first color produced by a corresponding one of the first light-emitting elements from a reference one.
The light-emitting element controlling circuit is responsive to the image input signal to produce reference pulse signals each for actuation of one of the first light-emitting elements for producing the light of the first color at a given luminance specified by the image input signal. Each of the reference pulse signals has a width specifying a duration of emission of the light from a corresponding one of the first light-emitting elements and a height that is a function of a value of current for excitation of the one of the first light-emitting elements. The light-emitting element controlling circuit stores therein correction factors each required to substantially compensate for a shift in chromaticity of the light emitted from one of first light-emitting elements from a reference one and produces correction pulse signals based on the correction factors each of which is applied to a corresponding one of said second light-emitting elements to produce the light of the second color, thereby minimizing the difference in chromaticity of the lights of the first color between the pixels.
The light-emitting element controlling circuit may produce the correction pulse signals each of which has a width substantially identical with the width of a corresponding one of the reference pulse signals and a height determined based on a corresponding one of the correction factors as a function of the shift in chromaticity of the light from the reference one.
The light-emitting element controlling circuit may alternatively produce the correction pulse signals each of which has a height substantially identical with the height of a corresponding one of the reference pulse signals and a width determined based on a corresponding one of the correction factors as a function of the shift in chromaticity of the light from the reference one.
The light-emitting element controlling circuit may output each of the correction pulse signals in a given time sequential relation to output of a corresponding one of the reference pulse signals.
The light-emitting element controlling circuit may also correct each of the reference pulse signals so as to substantially eliminate a difference between a luminance of the light emitted from a corresponding one of the first light-emitting elements and a target one.
The light-emitting element controlling circuit may produce the correction pulse signals based on the correction factors which are applied to said second and third light-emitting elements in a corresponding one of the pixels to produce the lights of the second and third colors, thereby visually shifting the light of the first color to the second and third colors to minimize the difference in chromaticity of the lights of the first color between the pixels.
The light-emitting element controlling circuit may correct each of the reference pulse signals so as to substantially eliminate a difference between a luminance of the light emitted from a corresponding one of the first light-emitting elements and a target one.
Each of the first color, the second color, and the third color is one of red, green, and blue.
Each of said first, second, and third light-emitting elements may be implemented by a light-emitting diode.